JPS598081B2 - Combined beam antenna for automotive radar - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a composite beam antenna for a radar device mounted on a vehicle, such as an automobile, to detect targets such as oncoming vehicles.

In radar devices that are attached to automobiles or other planar objects to detect targets such as oncoming vehicles, the antenna height and the target height are fixed substantially constant, resulting in the following problem.

That is, FIG. 1a shows the directivity of the antenna in free space, but if the antenna installation height h is low, it is likely to be affected by interference due to waves reflected from the ground surface.

For example, as shown in Figure b, the direct wave W1 and the reflected line W2 intersect at a point P, and the path length l1 of the direct wave W1 and the path 12 of the reflected wave W2 have a predetermined relationship with respect to the wavelength of the transmitted wave. When the two are in opposite phase, they cancel each other out, and even if there is a target at point P, there is no reflected wave from the target, and the received power becomes zero.

In reality, the direct wave W1 and the reflected wave W2 have different strengths, as is clear from the radiated electric field intensity distribution shown in Figure 1a, and the
1>W2, so complete cancellation is not performed, and the target object is not regarded as a point, but generally has various shapes of a certain size, so that the received power is only slightly dented.

Figure 2a shows the vertical plane radiation characteristics of the antenna affected by this ground reflected wave, and the horizontal axis shows the distance R from the antenna,
If the height h from the ground is plotted on the vertical axis, this radiation characteristic becomes a comb shape represented by a curve C1.

C1a, C1b...... are the depressed parts due to the influence of reflected waves, and the depth of these depressions depends on the directivity of the antenna in free space as mentioned above. Determined by etc.

Expressed mathematically, the vertical plane radiation characteristic E of the radar antenna
(θ) is as follows, where h1 is the height of the antenna above the ground.

Here, λ is the wavelength of the transmitted wave in free space, θ is the elevation angle, and Eo(θ) is the vertical in-plane radiation characteristic of the antenna in free space.

This E(θ) is illustrated as a curve C1 in FIG. 2a, and the angle θnull at which the electric field strength becomes zero and the angle θmaX at which the electric field strength becomes the maximum are expressed by equations (2) and (3), respectively.

However, n is an integer.

Therefore, if we consider the case where the target moves away from the radar antenna installation position at a height h2 above the ground, the radar received power P
r will change as shown by curve C2 in FIG. 2b.

That is, as the target moves away, that is, as the distance R increases, the received power Pr decreases by 1/R4 as a whole, and moreover, the directional concave portions C1a, Cxb in figure a...
・・・・・・・・・Then C2a, C2b・・・・・・
As shown in ・・・・・・・・・《Depressed at 1/R8.

If there is such a drop, the distance R corresponding to the relevant part
l t R2 J R3・・・・・・・・・・・・・・・
・Objects located in the area become undetectable or difficult to detect.

This is not a big problem in practice because the received power level itself is high at short distances, but at long distances the received power level itself decreases by 1/R4, and the drop width R
Since W becomes wider, it becomes a dominant factor in radar detection performance.

This is hardly a problem when the target moves in three dimensions, such as a flying object, because the height of the target changes freely. Congested areas create blind spots, so to speak.

On the other hand, in-vehicle radar requires a relatively sharp beam (2 to 5 degrees), a so-called pencil beam, in order to suppress unnecessary reflections from around the road and obtain only the reflected waves from the desired target. shall be.

The relationship between the beam width θHp and the aperture length D is shown in (4) below.
It is given by Eq.

θH −70°λ/D ・・・・・・・・・・・・・・・
・・・・・・・・・・・・(4)p As is clear from equation (4), in order to obtain a sharp beam, a large aperture area is required compared to the wavelength, but in-vehicle radar is equipped with It is not possible to install an antenna with a large aperture area.

Therefore, it is conceivable to either reduce the wavelength λ, that is, use a very high frequency, or to obtain an equivalently sharp beam by combining fan beams from linear wave sources with the same polarization.

However, increasing the frequency is subject to major hardware restrictions, so a composite beam system that obtains a pencil beam using a linear wave source with the same polarization is suitable for in-vehicle radar.

However, since the composite beam method naturally has a lower gain than a planar antenna with an equivalent beam, in-vehicle radars using this method are subject to the strong influence of interference caused by reflection from the ground surface as described above, which may cause a drop in received power. There is.

For radars with low ground clearance, this drop becomes a factor that directly controls the maximum detection distance.

For example, if the minimum receiving sensitivity Prmin of the radar is at the position shown in Fig. 2b, and is greater than the bottom value of the depression C2o, the maximum detection distance Rmax of this radar is the distance R2, which is expressed by equation (5). It will be done.

As is clear from Figure 2b, if the drop in received power at distance R2 can be compensated for, the maximum detection distance is R.
1, and increases dramatically.

As is clear from equation (5), in order to compensate for the drop in received power at a certain distance, some means must be used to make the radiation characteristic of the radar antenna corresponding to that distance concave at the radiation angle θ. All you have to do is to prevent this from occurring.

For this purpose, as previously proposed by the present applicant in Japanese Patent Application No. 50-98781, another radar antenna (only the transmitting antenna is sufficient) is installed at a height h'1, and the maximum beam direction of this antenna is It is sufficient to match the beam minimum direction of the radar antenna installed at the height h1 and to overlap both.

Height h/. is derived from equations (2) and (3), and is given by equation (6) regardless of frequency.

However, n is an integer.

Also, when the antennas are installed at heights h1 and h'1, the radiation characteristic E(θ) in the vertical plane is as follows.

However, Eo( is the vertical in-plane radiation characteristic in free space of an antenna installed at a height h/1.

When compensating for the above-mentioned depression C2c, n = 2 and 3, so from equation (6), if another 4 antenna is installed at the height of h1 = -h1, the received power will drop at distance R2. can be compensated.

However, in the case of vehicle radar, there are various restrictions such as not interfering with driving functions and not deteriorating the design, and it is extremely difficult to attach two planar antennas such as parabolas to a vehicle.

Therefore, in the present invention, we focused on the fact that the wave source can be linear in the fan beam composite beam system, and by taking advantage of this, we will provide a composite beam antenna that meets the mounting conditions of an in-vehicle radar antenna and has a long maximum detection distance. That is.

The present invention is an antenna for an on-vehicle radar device that detects a target whose height is approximately constant above the ground. The antenna group consisting of a set of antennas is installed at a height above the ground such that the maximum beam direction of one antenna overlaps the minimum beam direction of the other antenna, and each set is fed with in-phase power. This will be explained in detail below with reference to the drawings.

Figure 3 shows the original form of an antenna that forms a pencil beam by combining fan beams, and consists of a linear wave source, that is, a wire, bar, or narrow strip (for simplicity, we will refer to it as a linear element). It is constructed by arranging antenna A and antenna B in a cross shape.

This is Mill's cross antenna (Mil1sCress)
The radiation characteristic is a combination of the radiation characteristic Ra of antenna A and the radiation characteristic Rb of antenna B, as shown in FIG. 4, and a sharp composite beam 2 is obtained.

Such radiation characteristics can be achieved by arranging antenna A (or B) in a T-shape tilted at 90 degrees with its end touching the middle of antenna B (or A), as shown in Figures 5a and b.
Or, as shown in Figure 5C, both antennas A and B are
It can also be obtained by arranging it in a shape.

FIG. 6 shows an embodiment of the present invention, in which the antenna 1A
and antenna 1B are assembled in a T-shape as shown in Fig. 5a, and antenna 1A is used for transmitting and antenna 1B is used for receiving.
and parallel to the ground surface E, and at a height h'1 below this first antenna 1,
Similarly, T is transmitted by the transmitting antenna 2A and the receiving antenna 2B.
A second antenna 2 configured in a shape is arranged in the same direction as the first antenna 1.

The relationship between the heights h1 and h'1 of both antennas 1 and 2 is as shown in equation (6) above.

Further, both antennas 1 and 2 are connected to a transmitter/receiver (not shown) so as to feed power in the same phase.

In the vehicle-mounted radar antenna having such a configuration, a drop in received power caused by interference of reflected waves from the ground surface E is compensated for by the second antenna 2 having a predetermined height relationship with the first antenna 1. , the maximum detection distance increases dramatically to a sufficient distance for vehicles.

Also, since the wave source (antenna) that generates the fan beam is linear, the equivalent area of the first and second antennas 1 and 20 is wide as shown by the chain line, but the actual area occupied by the antenna is extremely small. Sufficient space is secured around the elements of the first and second antennas 1 and 2, and even if the antenna is mounted on the radiator grille of an automobile, ventilation is good and there is no risk of impairing engine cooling.

By the way, if a planar antenna such as a parabolic or horn-shaped antenna that generates an equivalent beam is installed at the same location, the entire installation area will be blocked, so even if a pencil beam is obtained with high gain, the amount of air intake will be reduced. It is significantly reduced and presents a serious hindrance to engine cooling.

7 to 9 each show an embodiment of the present invention.

Figure 7 uses the L-shaped structure shown in Figure 5e, and in this case as well, the transmitting antenna 1A and two people are installed parallel to the ground surface E and at predetermined heights h1 and h'1, respectively. Then, receiving antennas I B t 2 B are placed on both sides of the antennas.

That is, the wave source has a rectangular opening shape, that is, a rectangular frame shape, and has symmetry, and is a shape particularly suitable for automobiles and the like where left and right balance is important for aesthetic sense.

Figures 8 and 9 show the first and second antennas 1 and 2, except for one of the receiving antennas IB and 2B, in which the other is shared, parallel to the heights h1 and h'1 from the ground surface E. Transmitting antenna IA installed on 2A, receiving antenna IB on one side of 2A
(or 2B) to make the antenna group U-shaped (Fig. 8), or between the transmitting antennas IA and 2A, or if symmetry is particularly important, the receiving antenna I
Place B (or 2B) to form a cursive character (Figure 9).

In this way, one receiving antenna can be omitted, which reduces costs by saving materials and man-hours, and also reduces the installation area, giving the design a unique feel. be.

In particular, if the common receiving antenna IB (or 2B) is installed in the center of both transmitting antennas IA and 2A as shown in FIG. 9, the configuration will be symmetrical, and as shown in FIG. If it is installed in the center of the grille 11 surface, it can be installed at an appropriate location for detection operation and prevention of hazards due to aging of the driver's radiation.
It is also visually good.

As explained in detail above, according to the present invention, an in-vehicle radar that satisfies the installation conditions for an in-vehicle radar and can compensate as much as possible for the influence of reflected waves from the ground surface, etc., and can achieve a maximum detection distance of can provide composite beam antennas for

[Brief explanation of the drawings] Figure 1a is a diagram showing the directivity of the antenna in free space, Figure b is a diagram explaining the influence of the reflective liquid, and Figures 2a and b are diagrams showing the effect of the reflected wave. Fig. 3 is a configuration diagram explaining the original form of a composite beam type antenna, Fig. 4 is its radiation characteristic diagram, and Figures 5 a to c are diagrams of the combined beam type antenna. Block diagram showing a modified example of the antenna, No. 6
The figure is a block diagram showing one embodiment of a combined beam antenna for an on-vehicle radar according to the present invention, FIGS. 7 to 9 are block diagrams showing other embodiments of the present invention, and FIG. 10 is a block diagram showing an antenna according to the present invention. FIG. 1...First antenna, 2...Second antenna, IA, 2A...Transmission antenna, IB
, 2B...Receiving antenna, E...Ground surface.

Claims (1)

[Claims] 1. In an antenna for an on-vehicle radar device that detects a target whose height is approximately constant above the ground, an antenna consisting of a set of a plurality of linear wave sources that form a pencil beam with a fan beam of the same polarization is installed at a height of the vehicle. A group of antennas consisting of a plurality of antennas arranged side by side in the horizontal direction is installed at a height above the ground such that the maximum beam direction of one antenna overlaps the minimum beam direction of the other antenna, and A composite beam antenna for automotive radar that features in-phase power feeding. 2. An antenna with a set of linear wave sources is constructed by combining two linear antennas in a horizontal T-shape, and two sets of such horizontal T-shaped antennas are arranged in parallel in the vehicle height direction to form an antenna group. A combined beam antenna according to claim 1. 3. A patent claim characterized in that an antenna having a set of linear wave sources is constructed by combining two linear antennas in a horizontal L-shape, and two sets of such horizontal L-shaped antennas are arranged in a rectangular frame shape to form an antenna group. The range of combined beams described in item 1.
antenna. 4 A patent claim characterized in that the antenna group is constructed by combining three linear antennas in a U-shape, and the vertical receiving antenna at the end is shared by the upper and lower horizontal transmitting antennas. Combined beam antenna according to scope 1. 5. Claims characterized in that the antenna group is constructed by combining three linear antennas in a square shape, and the central vertical receiving antenna is shared by the upper and lower horizontal transmitting antennas. Combined beam antenna according to item 1.